If a person who is known for lying says "Let me be honest with you", should you believe him?

Generally, no. It may depend on what they are tallking about, but unless you can discern that they are attempting to correct a falsehood, you can’t trust them.

If a person who is known for lying says anything, I would proceed with caution.

I’m a poor judge of liars because I’m not one and tend to believe what people say. But if my trust in someone has been compromised, never mind exterminated, I’m not going to be very quick to forget.

However, I do see a gray area. A person who reschedules a lunch date claiming a migraine is not in the same class with, say, someone who accuses me of doing something that he himself did and I didn’t, or who makes up a false story in order to get me to give him money.

It’s a shame, but it has become inherently suspicious whenever anyone these days uses the word honest in a brief statement prior to making a particular point.
Instinctively puts anyone listening on the back foot in terms of believing sincerity, a word that has genuinely turned perverse.

Nope, in this case it may be an even bigger lie than the ones he/she is used to telling! The mother of all lies!’

No. The only time I would accept that kind of caveat is if I felt it was really necessary to tell someone something that might hurt their feelings, as in “I don’t want to have to say this but to be honest…”

I always loved the looks on Judge Judy’s face when people started out their sentences like that!

I despise those words due to the fact that I’ve NEVER heard them when a lie didn’t follow!!! Even when it’s a friend that I trust with my life due to their ability to be honest, when they start out “Let me be honest with you” I feel a chill go through my soul & I await the inevitable lie that will follow!!! I try to NEVER use those words myself; but, it does occasionally happen & I react exactly the same even when I know I’m telling the truth!!! My instincts automatically react “Ut-oh, here comes the lie”!!!

So to answer your question, NO I don’t trust a known liar when they tell me they’re being honest!!!

If someone tells a lie just once then they have blotted their copybook and everything they say becomes suspect. Fool me once shame on you, fool me twice shame on me.

No.

I’m suspicious of anyone who says, “Let me be honest…” The truth doesn’t need a preamble or qualifier.

No.
Ask them Why questions as a liar has trouble explaining .

I would think that “Let me be honest with you” would be the first best indication that a lie was forthcoming.
It is one of the 4 great lies, like;
“The check is in the mail.”
“It’ll be our little secret.”
“Trust me.”
“I won’t _______ in your ______.”

Depends on delivery.

Could very well be the opener for a big fat one. Or it could be that they’ve decided to talk about something that they weren’t too keen about talking about.

@kritiper Correct as a liar hopes that you will believe anything after saying that whereas an honest person does not have to ask one to trust them.

To be honest with you, I never trust anything anyone says who begins by saying “Let me be honest with you….”.

Its just a way to get you to listen.

Lying is a habit, or addiction. No liar stops overnight, nor has any remorse or reservation about saying, ‘let me be honest with you’ with the intent of lying